How to calculate the next/prev working day starting from a given date.
working days are monday to friday.
This article replaces the obsolete artice 2387

type
  // callback function for holidays
  THolidayCallback = function(const d:TDateTime):boolean;


function PrevWorkingDay(const date:TDateTime; callback:THolidayCallback):TDateTime;
begin
   Result := Int(date) - 1.0;  // yesterday 0:00

   if Assigned(callback) then
      while (DayOfWeek(Result) in [1,7]) or callback(Result) do
         Result := Result - 1.0
   else
      while (DayOfWeek(Result) in [1,7]) do
         Result := Result - 1.0;
end;

function NextWorkingDay(const date:TDateTime; callback:THolidayCallback):TDateTime;
begin
   Result := Int(date) + 1.0;  // tomorrow 0:00

   if Assigned(callback) then
      while (DayOfWeek(Result) in [1,7]) or callback(Result) do
         Result := Result + 1.0
   else
      while (DayOfWeek(Result) in [1,7]) do
         Result := Result + 1.0;
end;

===========================================================================

// example for the callback function

function IsHoliday(const d:TDateTime):boolean;
var
   year, month, day;
begin
   DecodeDate(d, year, month, day);
   if (month = 12) and ((day=25) or (day=26)) then
      result := True   // christmas
   else if (month=12) and (day=31) then
      result := True   // sylvester
   else if (month=1) (day=1) then
      result := True;

   // more queries here
   // e.g. a lookup in a database
end;